With the reason of receiving power from an alternating current (AC) power source, electrical loads such as commonly used electrical and electronic devices require electrical insulation for an output connected to the load. For this reason, a first side of a driving circuit, which is connected to the AC power source, is insulated from a second side of the driving circuit, which is connected to the load, by using a transformer of a power supply for controlling current. Then, the power supply for controlling current performs feedback control by transferring information on a current flowing through the load connected to the second side to the first side. In the past, it was not possible to directly feedback electrical signals due to the electrical insulation through the transformer. Therefore, an opto-coupler and a separate comparator have been used.
The prior power supply for controlling current shown in FIG. 1 rectifies an input AC voltage into a direct current (DC) voltage, and then transfers the rectified input DC voltage of the first side to the second side by using a flyback converter. The current flowing through a load line unit is converted into a voltage which is determined by a sense resistor, and then an output voltage of the comparator is obtained by integrating the voltage through a comparison of the voltage with a reference voltage of the comparator. The output voltage is reduced with the increase of the current flowing through the load line unit. As a result, a current flowing through the opto-coupler increases in proportion to the current flowing through the load line unit, so that a feedback voltage is increased. When the feedback voltage is increased, a switching controller reduces a time during which a current flows through the first coil. Contrarily, when the current flowing through the load line unit is reduced, the time during which the current flows through the first coil is increased, so that the current flowing through the load line unit is controlled to have a magnitude corresponding to that of the reference voltage.
The prior power supply for controlling current uses the opto-coupler and the additional comparator, so that it has a cost disadvantage. Particularly, in substitution for an incandescent lamp, it has a spatial limitation.